8.5's are More than enough for your mods even if you spray it.
You may even go slower with the 10.5's.
The best size slick is the smallest one you can get away with without spinning. A bigger slick is just more resistance.
Also take into account the diameter of the tire. Measure around the tire with a soft tape measure. This is a more important measurement than how tall they are.
Get a tire that compliments your gear for opimum max HP for RPM. You should be trapping at or a little over your peak HP/RPM.